The expert says search terms on Tamerlan Tsarnaev's laptop included "detonator, "transmitter and receiver" and "fireworks firing system." He says Dzhokhar Tsarnaev's laptop showed his computer activity focused heavily on Facebook and a Russian version of Facebook.

The testimony is part of an effort by Tsarnaev's lawyers to show that Tamerlan was the mastermind of the bombings. The defense admitted during opening statements that Dzhokhar participated in the attack but said he fell under the influence of the radicalized Tamerlan.

Tamerlan died following a shootout with police days after the bombings.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

Lawyers for Boston Marathon bomber Dzkhokhar Tsarnaev started their case by trying to show that his older brother was the driving force behind the 2013 terror attack.

The defense called one of its first witnesses on Monday. A cell site analyst testified that Tsarnaev's cell phone was being used in southeastern Massachusetts where he was attending the University of Massachusetts-Dartmouth while pressure cookers were being purchased north of Boston in Saugus 2 1/2 months before the bombing. The analyst also said large quantities of BBs were purchased a little over a month before the attack in two Wal-Mart stores in New Hampshire, at a time when Tsarnaev's cell phone was again being used near the university.

Twin bombs filled with BBs and shrapnel exploded near the marathon's finish line on April 15, 2013. Three people were killed and more than 260 injured.

At least three jurors cried and wiped their eyes with tissues as they looked at photos of the boy. His parents watched somberly from the second row of the courtroom.

Dr. Henry Nields, chief medical examiner for Massachusetts, said Martin suffered injuries to virtually every part of his body, including lacerations of his liver, left kidney and spleen, broken bones and third-degree burns. His stomach also was ruptured.

Tsarnaev's lawyer told the jury during opening statements that he participated in the bombings but that his older brother, Tamerlan, was the mastermind behind the attack. Prosecutors believe the brothers were seeking retaliation against the U.S. for wars in Muslim countries.

The case by defense lawyers is expected to be relatively short. Once that is complete, jurors will deliberate on whether Tsarnaev is guilty of the 30 federal charges against him.

During the second phase of the trial, the same jury will hear more evidence to decide whether Tsarnaev, 21, should be put to death or should spend the rest of his life in prison.